Semiconductor device having an improved electron transport property at a high electric field

ABSTRACT

In a semiconductor device having quantum wire structure formed by first and second semiconductor layers, the first and the second semiconductor layers are used as quantum well and quantum barrier layers, respectively. The quantum well layer has a first conduction band having a first Γ-valley and a first L-valley. The first Γ-valley has a first Γ-valley energy level. The first L-valley has a first L-valley energy level which is not lower than the first Γ-valley energy level. The quantum barrier layer has a second conduction band having a second energy level which is higher than the first L-valley energy level. The quantum wire structure is extended towards a predetermined direction. More particularly, the predetermined direction is parallel to a crystal orientation of (100).

BACKGROUND OF THE INVENTION

This invention relates to a semiconductor device for use in a fieldeffect transistor and, more particularly, to the semiconductor devicehaving a quantum wire structure.

It is known in the art that a semiconductor device comprises a quantumwire structure which includes a heterojunction structure formed betweena first semiconductor layer and a second semiconductor layer. Such asemiconductor device may be used as a field effect transistor which maybe called a high electron mobility transistor.

A conventional semiconductor device of the above-mentioned type isdisclosed in a title of "One-Dimensional Subbands and MobilityModulation in GaAs/AlGaAs Quantum Wires" contributed by K. Ismail et alto Applied Physics Letters, Vol. 54, March 1989, pages 1130 to 1132.

In the conventional semiconductor device disclosed by K. Ismail et al,an undoped GaAs layer is used as the first semiconductor layer while ann-type AlGaAs layer is used as the second semiconductor layer.

Herein, it is to be noted that the undoped GaAs layer has a firstconduction band consisting of three valleys which may be called a firstΓ-valley, a first L-valley, and a first X-valley.

Similarly, the n-type AlGaAs layer has a second conduction bandconsisting of three valleys which may be called a second Γ-valley, asecond L-valley, and a second X-valley.

Now, it will be assumed that the first Γ-valley, the first L-valley, andthe first X-valley have a first Γ-valley energy level, a first L-valleyenergy level, and a first X-valley energy level, respectively. Inaddition, it will be assumed that the second Γ-valley, the secondL-valley, and the second X-valley have a second Γ-valley energy level, asecond L-valley energy level, and a second X-valley energy level,respectively.

The first Γ-valley energy level is defined by an energy level of thebottom of the first Γ-valley. The first L-valley energy level is definedby an energy level of the bottom of the first L-valley. The firstX-valley energy level is defined by an energy level of the bottom of thefirst X-valley. The second Γ-valley energy level is defined by an energylevel of the bottom of the second Γ-valley. Similarly, the secondΓ-valley energy level is defined by an energy level of the bottom of thesecond Γ-valley. The second L-valley energy level is defined by anenergy level of the bottom of the second L-valley. The second X-valleyenergy level is defined by an energy level of the bottom of the secondX-valley.

In the conventional semiconductor device, the first Γ-valley has atleast one one-dimensional subband inasmuch as the first Γ-valley energylevel is lower than each of the second Γ-valley, the second L-valley,and the second X-valley energy levels so that the first Γ-valley has aquantum barrier in a heterojunction surface. On the other hand, theone-dimensional subband is not formed in each of the first L-valley andthe first X-valley inasmuch as each of the first L-valley and the firstX-valley energy levels is greater than at least one of the secondΓ-valley, the second L-valley, and the second X-valley energy levels. Asa result, electrons behave similar to electrons in a bulk semiconductordevice.

On applying a high electric field on the conventional semiconductordevice, electrons in the first Γ-valley are transited from the firstΓ-valley to the first L-valley. Inasmuch as the effective mass ofelectrons is greater in the first L-valley than that of electrons in thefirst Γ-valley, the average drift velocity of electrons decreases till adrift velocity defined by materials of the conventional semiconductordevice when the one-dimensional subband is not formed in the firstL-valley as described above. As a result, it is difficult to improve anelectron transport property at the high electric field in theconventional semiconductor device.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a semiconductordevice capable of improving an electron transport property at the highelectric field.

Other objects of this invention will become clear as the descriptionproceeds.

According to this invention, there is provided a semiconductor devicehaving a quantum wire structure formed by first and second semiconductorlayers. The quantum wire structure is extended towards a predetermineddirection. The first semiconductor layer comprises a first conductionband having a first Γ-valley and a first L-valley. The first Γ-valleyhas a first Γ-valley energy level. The first L-valley has a firstL-valley energy level which is not lower than the first Γ-valley energylevel. The second semiconductor comprises a second conduction bandhaving a second energy level which is higher than the first L-valleyenergy level. The predetermined direction is parallel to a prescribedcrystal orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a conventional semiconductor device;

FIG. 2 is a potential profile for describing a quantum wire structure ofthe semiconductor device illustrated in FIG. 1;

FIG. 3 is a diagram for illustrating a constant-energy surface in anL-valley of a semiconductor device having a diamond structure or azinc-blende structure as a crystalline structure;.

FIG. 4 is a diagram for illustrating relationship between a driftvelocity and a field strength;

FIG. 5 shows a perspective view of a semiconductor device according to afirst embodiment of this invention;

FIG. 6 is a potential profile for describing a quantum wire structure ofthe semiconductor device illustrated in FIG. 5;

FIG. 7 shows a perspective view of a semiconductor device according to asecond embodiment of this invention;

FIG. 8 is a potential profile for describing a quantum wire structure ofthe semiconductor device illustrated in FIG. 7;

FIG. 9 shows a perspective view of a semiconductor device according to athird embodiment of this invention;

FIG. 10 is a potential profile for describing a quantum wire structureof the semiconductor device illustrated in FIG. 9;

FIG. 11 shows a perspective view of a semiconductor device according toa fourth embodiment of this invention; and

FIG. 12 is a potential profile for describing a quantum wire structureof the semiconductor device illustrated in FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS: Prior Art:

Referring to FIG. 1, description will first be made as regards aconventional semiconductor device 10 for a better understanding of thisinvention. The semiconductor device 10 comprises a semi-insulatingsubstrate 11 which has a principal surface 12 directed upwards in FIG.1, a first semiconductor layer 13 on the principal surface 12, and asecond semiconductor layer 14 on the first semiconductor layer 13. Inthe example being illustrated, the semi-insulating substrate 11 iscomposed of GaAs. The first semiconductor layer 13 may be called abuffer layer which is composed of an undoped GaAs. The secondsemiconductor layer 14 comprises a spacer film 15 and an electronsupplying layer 16 on the spacer film 15. The spacer film 15 is composedof an undoped Al₀.3 Ga₀.7 As. The electron supplying layer 16 iscomposed of an n-type Al₀.3 Ga₀.7 As.

The electron supplying layer 16 is partially etched so that the electronsupplying layer 16 has a projection portion 16a directed upwards inFIG. 1. The projection portion 16a has a wire structure therein. A gateelectrode 17 is formed on the electron supplying layer 16 byevaporation.

In the illustrated example, a quantum wire structure is formed betweenthe first semiconductor layer 13 and the second semiconductor layer 14and includes a heterojunction structure.

On applying a voltage to the gate electrode 17, a one-dimensionalelectron gas (1DEG) 18 is generated in the first semiconductor layer 13near the surface of the first semiconductor layer 13 and under theprojection portion 16a to be used as a channel.

Referring to FIG. 2 in addition to FIG. 1, description will proceed to apotential energy in relation to a direction perpendicular to aheterojunction surface near 1DEG 18. The first semiconductor layer 13has a first conduction band comprising a first Γ-valley, a firstL-valley, and a first X-valley. Similarly, the second semiconductorlayer 14 has a second conduction band comprising a second Γ-valley, asecond L-valley, and a second X-valley.

The first Γ-valley has a first Γ-valley potential energy level definedby an energy level of the bottom of the first Γ-valley and depicted byE.sub.Γ^(W) in FIG. 2. The first L-valley has a first L-valley potentialenergy level defined by an energy level of the bottom of the firstL-valley and depicted by E_(L) ^(W). The first X-valley has a firstX-valley potential energy level defined by an energy level of the bottomof the first X-valley and depicted by E_(X) ^(W). Similarly, the secondΓ-valley has a second Γ-valley potential energy level defined by anenergy level of the bottom of the second Γ-valley and depicted byE.sub.Γ^(B) in FIG. 2. The second L-valley and the second X-valley havesecond L-valley and the second X-valley potential energy levels depictedby E_(L) ^(B) and E_(X) ^(B), respectively. The second L-valleypotential energy level is defined by an energy level of the bottom ofthe second L-valley. The second X-valley potential energy level isdefined by an energy level of the bottom of the second X-valley. Thesecond Γ-valley, the second L-valley, and the second X-valley potentialenergy levels may be collectively called a second potential energylevel.

As shown in FIG. 2, the first L-valley potential energy level is higherin 0.28 eV than the first Γ-valley potential energy level. The firstX-valley potential energy level is higher in 0.19 eV than the firstL-valley potential energy level. The second L-valley potential energylevel is higher in 0.1 eV than the second Γ-valley potential energylevel. The second X-valley potential energy level is higher in 0.05 eVthan the second L-valley potential energy level. Furthermore, it isnoted that the first Γ-valley potential energy level is lower in 0.24 eVthan the second Γ-valley potential energy level at the heterojunctionsurface. Namely, the first Γ-valley has a quantum barrier of 0.24 eV atthe heterojunction surface.

As readily understood in FIG. 2, the first Γ-valley has at least oneone-dimensional subband because the first Γ-valley potential energylevel is lower than the second potential energy level. Moreparticularly, the first Γ-valley has a pair of one-dimensional subbandsof which levels are depicted by reference numbers 18a and 18b,respectively. In the one-dimensional subband having the level depictedby 18a, electrons have a wave function depicted by a reference number19a. In the one-dimensional subband having the level depicted by 18b,electrons have a wave function depicted by a reference number 19b.Inasmuch as the second Γ-valley potential energy level is lower thaneach of the first L-valley and the first X-valley potential energylevels, the one-dimensional subband is not formed in each of the firstL-valley and the first X-valley.

Attention will be directed to the quantum wire structure. As describedabove, the first Γ-valley has at least one one-dimensional subband. Inthe illustrated example, the one-dimensional subband in the firstΓ-valley will be called a first one-dimensional subband.

It will be assumed that the quantum structure has a rectangle in a crosssection. The rectangle has a width of Ly and a thickness of Lz. Thepotential energy level ε_(mn) of the first one-dimensional subband isgiven by:

    ε.sub.mn ={η.sup.2 (h/2η).sup.2 /2m*Γ}{(m/Ly).sup.2 +(n/Lz).sup.2 },                                          (1)

where m represents quantum number relative to the direction of thewidth. n represents quantum number relative to the direction of thethickness. m*Γ represents the effective mass of electron in the firstprimary Γ-valley. h represents Planck constant.

Now, it will be assumed that each of the width Ly and the thickness Lzis equal to 150 angstroms. When m*Γ is 0.067m₀, where m₀ represents amass of a free electron, a ground level is represented by ε₁₁ whichbecomes to 50 meV. A first excitation level is represented by ε₁₂ or ε₂₁which becomes to 120 meV. Inasmuch as the first excitation level isgreater in 70 meV than the ground level, electrons transit from groundlevel to the first excitation level when the electric field is appliedto the quantum wire structure at several hundreds of volts percentimeter (V/cm). Inasmuch as the first L-valley potential energy levelis higher in 230 meV than the ground level, electrons transit from thefirst Γ-valley to the first L-valley when the electric field is appliedto the quantum wire structure at several thousands of V/cm.

As described above, the effective mass of electron becomes great in thefirst L-valley. As a result, electron has a low velocity in the firstL-valley.

As readily understood from the above description, electrons aretransited from the first Γ-valley to the first L-valley when thesemiconductor device 10 is supplied with a very high electric field.Inasmuch as the effective mass of electron becomes great in the firstL-valley as described above, quantum limit is not materialized so thatthe drift velocity of electrons decreases to a saturation velocity whichis defined by materials of the first and the second semiconductorlayers. Namely, it is difficult to improve an electron transportproperty at very high electric field in the semiconductor device 10.

Principle of the Invention:

Description will proceed to a principle of this invention.

In general, the quantum wire structure is formed by the firstsemiconductor layer and the second semiconductor layer on the firstsemiconductor layer. The first semiconductor layer serves as a quantumwell layer. The second semiconductor layer serves as a quantum barrierlayer.

Now, it will be assumed that the first Γ-valley potential energy levelis represented by E_(C1). It will be assumed that the first L-valleypotential energy level is represented by E_(L1). It will be assumed thatthe second potential energy level is represented by E_(C2).

Let electron affinity be represented by χ₁ in the quantum well layer.Let electron affinity be represented by χ₂ in the quantum barrier layer.Let an energy gap be represented by ΔE_(L1) between the first Γ-valleypotential energy level and the first L-valley potential energy level.According to Anderson model, the first Γ-valley potential energy level,the first L-valley potential energy level, and the second potentialenergy level are respectively given by: ##EQU1##

As described above, it is necessary to make the first L-valley potentialenergy level be lower than the second potential energy level in order toimprove the electron transport property at the high electric field.Namely, it is necessary to obtain Inequality given by:

    E.sub.c1 ≦E.sub.L1 <E.sub.c2                        (3)

By use of Equation (2), Inequality (3) is rewritten into Inequalitygiven by:

    0≦ΔE.sub.L1 21 χ.sub.1 -χ.sub.2       (4)

It is noted that the first L-valley has at least one one-dimensionalsubband when a quantum well structure is formed by the use of materialssatisfying Inequality (4). In this case, the quantum well structure hasa sectional size whose dimension is approximately equal to de Brogliewavelength of electron.

The quantum wire structure is extended towards a predetermineddirection. The predetermined direction is parallel to a prescribedcrystal orientation which may be the crystal orientation of (100). In acase where the predetermined direction is parallel to the crystalorientation of (100), electrons are able to move to only the crystalorientation of (100) in each of the first Γ-valley and the firstL-valley with restraining electrons from moving towards a directionperpendicular to the crystal orientation of (100) in the one-dimensionalsubband of each of the first Γ-valley and the first L-valley. On theother hand, the scattered electrons move at random in the first L-valleyas known in the art.

Referring to FIG. 3, it will be assumed that a semiconductor has eitherone of a diamond structure and a zinc-blende structure in a crystallinestructure. When the semiconductor has either one of the diamondstructure and the zinc-blende structure in the crystalline structure, aconstant-energy surface is illustrated in the first L-valley as depictedby a reference numeral 19c. In a space defined by momentum, theconstant-energy surface forms a spheroid having a maximum effect mass inthe crystal orientation of (111). When electrons move to the crystalorientation of (100), the effective mass of electrons is represented asa conductivity effective mass m*_(LC) given by:

    m*.sub.LC =3(2/m*.sub.Lt +1/m*.sub.L1).sup.-1,             (5)

where m*_(Lt) represents a transversal effective mass of electrons inthe first L-valley. m*_(L1) represents a longitudinal effective mass ofelectrons in the first L-valley.

When electrons move at random, the effective mass of electrons isrepresented as a density-of-state effective mass m*_(LD) given by:

    m*.sub.LD =(m*.sub.L1 ·m*.sub.Lt.sup.2).sup.1/3   (6)

When the quantum well layer is composed of GaAs, the transversaleffective mass becomes to 0.075m₀, where m₀ represents a mass of a freeelectron. The longitudinal effective mass becomes to 1.9m₀.

Inasmuch as the transversal effective mass is not equal to thelongitudinal effective mass as described above, the density-of-stateeffective mass is greater than the conductivity effective mass. When thequantum well layer is composed of GaAs, the conductivity effective massand the density-of-state effective mass become to 0.11m₀ and 0.22m₀,respectively. Namely, the conductivity effective mass is equal to a halfof the density-of-state effective mass.

In the quantum wire structure having the quantum well layer composed ofGaAs, calculation is carried out to obtain a relationship between adrift velocity and a field strength by the use of Monte Carlo method asshown by a solid line in FIG. 4. For convenience of description,calculation is carried out in a bulk semiconductor composed of GaAs toobtain a relationship between a drift velocity and a field strength bythe use of Mote Carlo method as shown by a broken line in FIG. 4.

As readily understood from FIG. 4, it is possible to improve theelectron transport property in the semiconductor device having thequantum wire structure according to this invention when the quantum wirestructure is extended towards the crystal orientation of (100) and whenthe one-dimensional subband is formed in the first L-valley.

Embodiments:

Referring to FIG. 5, description will proceed to a semiconductor device20 of a first embodiment according to this invention. The illustratedsemiconductor device 20 comprises a semi-insulating substrate 21 whichhas a principal surface 22 directed upwards in FIG. 5 and a buffer layer23 on the principal surface 22. The semiconductor device 20 furthercomprises an electron supplying layer 24 and a quantum well layer 25.

In the example being illustrated, the semi-insulating substrate 21 iscomposed of GaAs and has the crystal surface of (001). The buffer layer23 is composed of GaAs which is undoped. It is noted that the electronsupplying layer 24 comprises first through third supplying films 24a to24c each of which is composed of an n-type AlGaAs. The quantum welllayer 25 is composed of undoped InGaAs and is interposed among the firstthrough the third supplying films 24a to 24c. The quantum well layer 25has a quantum wire structure of which the orientation is parallel to thecrystal orientation of (100).

On manufacturing the semiconductor device 20 illustrated in FIG. 5, thesemi-insulating substrate 21 is prepared which has a front surface and arear surface directed upwards and downwards in FIG. 5, respectively.

The buffer layer 23 is formed or deposited on the principal surface 22to the thickness of 1 μm by the use of Molecular Beam Epitaxy (MBE).After deposition of the buffer layer 23, a first film or layer isdeposited on the buffer layer 23 to the thickness of 350 angstroms byMBE. The first film is composed of an n-type Al₀.45 Ga₀.55 As of which adoping density is equal to 3×10¹⁸ /cm³. Sequentially, a second film isdeposited on the first film to the thickness of 100 angstroms by MBE.The second film is composed of an undoped In₀.2 Ga₀.8 As. A third filmis deposited on the second film by MBE to the thickness of 200angstroms. The third film is composed of an n-type Al₀.45 Ga₀.55 As ofwhich a doping density is equal to 3×10¹⁸ /cm³.

With Electron Beam lithography, the first through the third films areetched by the use of a mask which has a predetermined pattern. The firstfilm is etched into the first supplying film 24a having a plate portion241 and a projection portion 242 on a center of the plate portion 241.The second film is etched into the quantum well layer 25 on theprojection portion 242. The third film is etched into the secondsupplying film 24b on the quantum well layer 25. As described above, thequantum well layer 25 has the quantum wire structure of which thecrystal orientation is parallel to (100) orientation and of which thewidth is 100 angstroms.

After etching, the third supplying film 24c is formed by MBE so as tocover the first and the second films 24a and 24b and the quantum welllayer 25. The third supplying film 24c is composed of an n-type Al₀.45Ga₀.55 As of which a doping density is equal to 3×10¹⁸ /cm³.

When applying the electric field to the semiconductor device 20, theone-dimensional electron gas is generated in the quantum well layer 25and will be designated by a reference numeral 26 in FIG. 5.

The quantum well layer 25 has the first conduction band comprising thefirst Γ-valley, the first L-valley, and the first X-valley.Consequently, the quantum well layer 25 serves as the firstsemiconductor layer. On the other hand, the electron supplying layer 24has the second conduction band comprising the second Γ-valley, thesecond L-valley, and the second X-valley. Therefore, the electronsupplying layer 24 serves as the second semiconductor layer.

Referring to FIG. 6, description will proceed to a potential energylevel in relation to a direction perpendicular to the quantum wirestructure of the semiconductor device 20 illustrated in FIG. 5. In FIG.6, E.sub.Γ^(W) and E_(L) ^(W) represent the first Γ-valley potentialenergy level and the first L-valley potential energy level,respectively, as described in conjunction with FIG. 2. Similarly,E.sub.Γ^(B), E_(L) ^(B), and E_(X) ^(B) represent the second Γ-valleypotential energy level, the second L-valley potential energy level, andthe second X-valley potential energy level, respectively.

Now, let m*Γ be 0.054m₀ in Equation (1). Inasmuch as each of the widthand the thickness is equal to 100 angstroms in the quantum wirestructure, the ground level ε₁₁ becomes to 0.14 eV in the firstΓ-valley. Each of the first excitation levels ε₁₂ and ε₂₁ becomes to0.35 eV.

As shown in FIG. 6, the first Γ-valley potential energy level is lowerin 0.39 eV than the first L-valley potential energy level. Each of thesecond Γ-valley, the second L-valley, and the second X-valley potentialenergy levels is approximately equal to one another. The first Γ-valleypotential energy level is lower in 0.55 eV than the second Γ-valleypotential energy level at the heterojunction surface between the quantumwell layer 25 and the electron supplying layer 24.

Accordingly, the first Γ-valley has at least one one-dimensionalsubband. In the example being illustrated in FIG. 6, the first Γ-valleyhas a pair of one-dimensional subbands of which levels are depicted byreference numbers 31 and 32, respectively. In the one-dimensionalsubband having the level depicted by 31, electrons have a wave functiondepicted by a reference number 33. In the one-dimensional subband havingthe level depicted by 32, electrons have a wave function depicted by areference number 34.

Similarly, the first L-valley has at least one one-dimensional subbandinasmuch as the first L-valley potential energy level is less in 0.16 eVthan the second potential energy level. In the example being illustratedin FIG. 6, the first L-valley has a pair of one-dimensional subbands ofwhich levels are depicted by reference numbers 35 and 36, respectively.In the one-dimensional subband having the level depicted by 35,electrons have a wave function depicted by a reference number 37. In theone-dimensional subband having the level depicted by 36, electrons havea wave function depicted by a reference number 38.

As readily understood from the above description, the electron supplyinglayer 24 serves as a quantum barrier layer in the first embodiment.

Now, it will be assumed that quantum well layer 25 is composed of In_(x)Ga_(1-x) As and the electron supplying layer 24 is composed of Al_(y)Ga_(1-y) As, where x represents a variable between zero and one (bothexclusive) and y represents a variable between zero and one (bothexclusive). The electron affinity becomes to a minimum value when thevariable y is equal to 0.45 in Al_(y) Ga_(1-y) As. When the variable yis not less than 0.35 and is not greater than 0.55, Equation (3) issatisfied with having no concern with the variable x. It is desirablefor the variable x to be large in order to effectively improve theelectron transport property in the semiconductor device inasmuch as theelectron affinity monotonously increases as the variable x increases inIn_(x) Ga_(1-x) As. However, the variable x is limited by a criticallayer thickness inasmuch as the lattice constant of InGaAs is differentfrom that of AlGaAs.

In Equation (1), it will be assumed that the width Ly and the thicknessLz are equal to each other. When each of the width Ly and the thicknessLz is equal to 50 angstroms, the ground state level is estimated as 0.56eV in the first Γ-valley in case where m*Γ is equal to 0.054m₀. Wheneach of the width Ly and the thickness Lz is equal to 100 angstroms, theground state level is estimated as 0.14 eV in the first Γ-valley in casewhere m*Γ is equal to 0.054m₀.

As described above in conjunction with FIG. 6, the first L-valleypotential energy level is greater in 0.39 eV than the first Γ-valleypotential energy level when the quantum well layer 25 is composed ofIn₀.2 Ga₀.8 As. Therefore, the thickness should not be less than 100angstroms in the quantum well layer 25 to make the semiconductor devicemaintain a one-dimensional confinement and to prevent the semiconductordevice from an indirect transition type. In the quantum well layercomposed of In_(x) Ga_(1-x) As, a misfit dislocation occurs in thequantum well layer 25 when the thickness of the quantum well layer 25 is150 angstroms at the variable x=0.2. The misfit dislocation occurs inthe quantum well layer 25 when the thickness of the quantum well layer25 is 80 angstroms at the variable x=0.3. Accordingly, the variable x isselected within a value which is not greater than 0.3.

Referring to FIG. 7, description will proceed to a semiconductor deviceof a second embodiment according to this invention. The illustratedsemiconductor device is different in structure from the semiconductordevice 20 illustrated in FIG. 5 and is therefore designated afresh by areference numeral 40. The semiconductor device 40 comprises thesemi-insulating substrate 21 which is described in conjunction with FIG.5. The semiconductor device 40 comprises the buffer layer, the electronsupplying layer, and the quantum well layer each of which is differentin composition from the semiconductor device 20. Therefore, the bufferlayer, the electron supplying layer, and the quantum well layer of thesemiconductor device 40 will be designated by reference numerals 41, 42,and 43, respectively. The semiconductor device 40 further comprises asuperlattice layer 44 between the semi-insulating substrate 21 and thebuffer layer 41.

In the example being illustrated, the superlattice layer 44 is composedof undoped GaSb/AlSb. The buffer layer 41 is composed of undoped AlGaSb.It is noted that the electron supplying layer 42 comprises first throughthird supplying films 42a to 42c each of which is composed of an n-typeAlSb. The quantum well layer 43 is composed of undoped InAs and isinterposed among the first through the third supplying films 42a to 42c.The quantum well layer 43 has a quantum wire structure of which thecrystal orientation is parallel to (100) orientation.

On manufacturing the semiconductor device 40 illustrated in FIG. 7, thesemi-insulating substrate 21 is prepared which has a front surface and arear surface directed upwards and downwards in FIG. 7, respectively.

The superlattice layer 44 is formed on the principal surface 22 by theuse of Molecular Beam Epitaxy (MBE). More particularly, the superlatticelayer 44 comprises a plurality of film pairs. In the illustratedexample, the number of film pairs is equal to twenty. Each of the filmpairs has primary and secondary films. The primary film is composed ofundoped GaSb and has the thickness of 40 angstroms. The secondary filmis composed of undoped AlSb and has the thickness of 40 angstroms. Thesecondary film is formed on the primary film.

The buffer layer 41 of undoped Al₀.7 Ga₀.3 Sb is deposited or formed onthe superlattice layer 44 to the thickness of 1000 angstroms by MBE.After deposition of the buffer layer 41, a first film is deposited onthe buffer layer 41 to the thickness of 200 angstroms by MBE. The firstfilm is composed of an n-type AlSb of which a doping density is equal to3×10¹⁸ /cm³. Sequentially, a second film is deposited on the first filmto the thickness of 80 angstroms by MBE. The second film is composed ofan undoped InAs. A third film is deposited on the second film by MBE tothe thickness of 200 angstroms. The third film is composed of an n-typeAlSb of which a doping density is equal to 3×10¹⁸ /cm³.

With Electron Beam lithography, the first through the third films areetched by the use of a mask which has a predetermined pattern. The firstfilm is etched into the first supplying film 42a having a plate portion421 and a projection portion 422 on a center of the plate portion 421.The second film is etched into the quantum well layer 43 on theprojection portion 422. The third film is etched into the secondsupplying film 42b on the quantum well layer 43. As described above, thequantum well layer 43 has the quantum wire structure of which thecrystal orientation is parallel to (100) orientation and of which thewidth is 160 angstroms.

After etching, the third supplying film 42c is formed by MBE so as tocover the first and the second supplying films 42a and 42b and thequantum well layer 43. The third supplying film 24c is composed of ann-type AlSb of Which a doping density is equal to 3×10¹⁸ /cm³.

In the semiconductor device 40, the lattice constant of thesemi-insulating substrate 21 is different from that of the buffer layer41. Inasmuch as the superlattice layer 44 is located between thesemi-insulating substrate 21 and the buffer layer 41, the superlatticelayer 44 serves to dissolve a lattice mismatch between thesemi-insulating substrate 21 and the buffer layer 41.

When applying the electric field to the semiconductor device 40, theone-dimensional electron gas is generated in the quantum well layer 43and will be designated by a reference numeral 45 in FIG. 7.

As readily understood from description in conjunction with FIG. 7, thequantum well layer 43 serves as the first semiconductor layer. Theelectron supplying layer 42 serves as the second semiconductor layer.

Referring to FIG. 8, description will proceed to a potential energylevel in relation to a direction perpendicular to the quantum wirestructure of the semiconductor device 40 illustrated in FIG. 7. In FIG.8, E.sub.Γ^(W), E_(L) ^(W), and E_(X) ^(W) represent the first Γ-valley,the first L-valley, and the first X-valley potential energy levels,respectively, as described in conjunction with FIG. 2. Similarly,E.sub.Γ^(B), E_(L) ^(B), and E_(X) ^(B) represent the second Γ-valley,the second L-valley, and the second X-valley potential energy levels,respectively.

Now, let m*Γ be 0.027m₀ in Equation (1). Inasmuch as the width Ly andthe thickness Lz is equal to 80 angstroms and 160 angstroms in thequantum wire structure, respectively, the ground level ε₁₁ becomes to0.27 eV in the first Γ-valley. The first excitation level ε₁₂ becomes to0.41 eV in the first Γ-valley.

As shown in FIG. 8, the first Γ-valley potential energy level is lowerin 0.69 eV than the first L-valley potential energy level. The firstL-valley potential energy level is lower in 0.75 eV than the firstX-valley potential energy level. On the other hand, the second Γ-valleypotential energy level is higher in 0.42 eV than the second L-valleypotential energy level. The second L-valley potential energy level ishigher in 0.17 eV than the second X-valley potential energy level.Furthermore, the first Γ-valley potential energy level is lower in 1.38eV than the second X-valley potential energy level at the heterojunctionsurface between the quantum well layer 43 and the electron supplyinglayer 42.

Accordingly, the first Γ-valley has at least one one-dimensionalsubband. In the example being illustrated in FIG. 8, the first Γ-valleyhas a pair of one-dimensional subbands of which levels are depicted byreference numbers 51 and 52, respectively. In the one-dimensionalsubband having the level depicted by 51, electrons have a wave functiondepicted by a reference number 53. In the one-dimensional subband havingthe level depicted by 52, electrons have a wave function depicted by areference number 54.

Similarly, the first L-valley has at least one one-dimensional subbandinasmuch as the first L-valley potential energy level is less than thesecond potential energy level. In the example being illustrated in FIG.8, the first L-valley has a pair of one-dimensional subbands of whichlevels are depicted by reference numbers 55 and 56, respectively. In theone-dimensional subband having the level depicted by 55, electrons havea wave function depicted by a reference number 57. In theone-dimensional subband having the level depicted by 56, electrons havea wave function depicted by a reference number 58.

As readily understood from the above description, the electron supplyinglayer 42 serves as a quantum barrier layer in the second embodiment.

Now, it will be assumed that the electron supplying layer 42 is composedof Al_(y) Ga_(1-y) Sb. Equation (3) is satisfied with having no concernwith the variable y. It is desirable to make the variable y be one inorder to effectively improve the electron transport property in thesemiconductor device 40 inasmuch as the electron affinity monotonouslydecreases as the variable x increases in Al_(y) Ga_(1-y) Sb.

Referring to FIG. 9, description will proceed to a semiconductor deviceof a third embodiment according to this invention. The illustratedsemiconductor device is different in structure from the semiconductordevice 20 illustrated in FIG. 5 and is therefore designated afresh by areference numeral 60. The semiconductor device 60 comprises thesemi-insulating substrate 21 and the buffer layer 23 each of which isdescribed in conjunction with FIG. 5. The semiconductor device 60comprises an electron supplying layer and a quantum well layer each ofwhich is different in composition from the semiconductor device 20.Therefore, the electron supplying layer and the quantum well layer ofthe semiconductor device 60 will be designated by reference numerals 61and 62, respectively.

In the example being illustrated, it is noted that the electronsupplying layer 61 comprises first and second supplying films 61a and62b each of which is composed of undoped InAlGaP. The quantum well layer62 is composed of undoped GaAs and is interposed between the firstsupplying film 61a and the second supplying film 61b. Each of the firstand the second, supplying films 61a and 61b has a thin film as shown bya broken line in FIG. 9. The thin film is composed of silicon and may becalled a δ doped Si film. The δ doped Si film in the first supplyingfilm 61a will be designated by a reference numeral 61c. The δ doped Sifilm in the second supplying film 61b will be designated by a referencenumeral 61d. The quantum well layer 62 has a quantum wire structure ofwhich the crystal orientation is parallel to (100) orientation.

On manufacturing the semiconductor device 60 illustrated in FIG. 9, thesemi-insulating substrate 21 is prepared which has a front surface and arear surface directed upwards and downwards in FIG. 9, respectively.

The buffer layer 23 is formed on the principal surface 22 to thethickness of 1 μm by the use of Molecular Beam Epitaxy (MBE). A firstfilm is deposited on the buffer layer 23 to the thickness of 1000angstroms by MBE. The first film is composed of undoped In₀.49 (Al₀.7Ga₀.3)₀.51 P. Sequentially, a second film of silicon is formed on thefirst film by MBE at a sheet density of 1×10¹³ /cm². A third film ofundoped In₀.49 (Al₀.7 Ga₀.3)₀.51 P is deposited on the second film byMBE to the thickness of 50 angstroms. A fourth film of undoped GaAs isdeposited on the third film by MBE to the thickness of 150 angstroms. Afifth film of undoped In₀.49 (Al₀.7 Ga₀.3)₀.51 P is deposited on thefourth film by MBE to the thickness of 50 angstroms. A sixth film ofsilicon is formed on the fifth film by MBE at a sheet density of 1×10¹³/cm². A seventh film is deposited on the sixth film by MBE to thethickness of 200 angstroms.

With Electron Beam lithography, the first through the seventh films areetched by the use of a mask which has a prescribed pattern. The firstthrough the third films are etched into the first supplying film 61ahaving a plate portion 611 and a projection portion 612 on a center ofthe plate portion 611. The fourth film is etched into the quantum welllayer 62 on the projection portion 612. The fifth through the seventhfilms are etched into the second supplying film 61b on the quantum welllayer 62. As described above, the quantum well layer 62 has the quantumwire structure of which the crystal orientation is parallel to (100)orientation and of which the width is 150 angstroms.

When applying the electric field to the semiconductor device 60, theone-dimensional electron gas is generated in the quantum well layer 62and will be designated by a reference numeral 63 in FIG. 9.

As readily understood from description in conjunction with FIG. 9, thequantum well layer 62 serves as the first semiconductor layer. Theelectron supplying layer 61 serves as the second semiconductor layer.

Referring to FIG. 10, description will proceed to a potential energylevel in relation to a direction perpendicular to the quantum wirestructure of the semiconductor device 60 illustrated in. FIG. 9. In FIG.10, E.sub.Γ^(W), E_(L) ^(W), and E_(X) ^(W) represent the firstΓ-valley, the first L-valley, and the first X-valley potential energylevels, respectively, as described in conjunction with FIG. 2. SimilarlyE.sub.Γ^(B), E_(L) ^(B), and E_(X) ^(B) represent the second Γ-valley,the second L-valley, and the second X-valley potential energy levels,respectively.

Now, let m*Γ be 0.067m₀ in Equation (1). Inasmuch as each of the widthand the thickness is equal to 150 angstroms in the quantum wirestructure, the ground level ε₁₁ becomes to 0.05 eV in the firstΓ-valley. Each of the first excitation levels ε₁₂ and ε₂₁ becomes to0.12 eV in the first Γ-valley.

As shown in FIG. 10, the first Γ-valley potential energy level is lowerin 0.28 eV than the first L-valley potential energy level. Each of thesecond Γ-valley, the second L-valley, and the second X-valley potentialenergy levels is approximately equal to one another. The first Γ-valleypotential energy level is lower in 0.37 eV than the second Γ-valleypotential energy level at the heterojunction surface between the quantumwell layer 62 and the electron supplying layer 61.

Accordingly, the first Γ-valley has at least one one-dimensionalsubband. In the example being illustrated in FIG. 10, the first Γ-valleyhas a pair of one-dimensional subbands of which levels are depicted byreference numbers 71 and 72, respectively. In the one-dimensionalsubband having the level depicted by 71, electrons have a wave functiondepicted by a reference number 73. In the one-dimensional subband havingthe level depicted by 72, electrons have a wave function depicted by areference number 74.

Similarly, the first L-valley has at least one one-dimensional subbandinasmuch as the first L-valley potential energy level is lower than thesecond potential energy level. In the example being illustrated in FIG.10, the first L-valley has a pair of one-dimensional subbands of whichlevels are depicted by reference numbers 75 and 76, respectively. In theone-dimensional subband having the level depicted by 75, electrons havea wave function depicted by a reference number 77. In the15.one-dimensional subband having the level depicted by 76, electronshave a wave function depicted by a reference number 78.

As readily understood from the above description, the electron supplyinglayer 61 serves as a quantum barrier layer in the third embodiment.

Now, it will be assumed that the electron supplying layer 61 is composedof In₀.49 (Al_(z) Ga_(1-z))₀.51 P, where z represents a variable betweenzero and one (both exclusive). Equation (3) is satisfied when thevariable z is not less than 0.4. Inasmuch as the electron affinitybecomes to a minimum value when the variable z is equal to 0.7 in In₀.49(Al_(z) Ga_(1-z))₀.51 P, it is desirable to make the variable z be about0.7 in order to effectively improve the electron transport property inthe semiconductor device 60.

Referring to FIG. 11, description will proceed to a semiconductor deviceof a fourth embodiment according to this invention. The illustratedsemiconductor device is different in structure from the semiconductordevice 20 illustrated in FIG. 5 and is therefore designated afresh by areference numeral 80.

The semiconductor device 80 comprises a semi-insulating substrate 81which has a principal surface 82 directed upwards in FIG. 11 and abuffer layer 83 on the principal surface 82. The semiconductor device 80further comprises an electron supplying layer 84, a spacer layer 85, anda quantum well layer 86.

In the example being illustrated, the semi-insulating substrate 81 iscomposed of InP and has the crystal surface of (001). The buffer layer83 is composed of InAlAs which is undoped. It is noted that the electronsupplying layer 84 comprises first and second supplying films 84a and84b each of which is composed of an n-type InAlAs. Furthermore, thespacer layer 85 comprises first and second spacer films 85a and 85b eachof which is composed of undoped InAlAs. The quantum well layer 86 iscomposed of undoped InGaAs and is interposed between the first spacerfilm 85a and the second spacer film 85b. The first supplying film 84a isformed on the buffer layer 83. The first spacer film 85a is formed onthe first supplying film 84a. In addition, the second supplying film 84bis formed on the second spacer film 85b. The quantum well layer 86 has aquantum wire structure of which the crystal orientation is parallel to(100) orientation.

On manufacturing the semiconductor device 80 illustrated in FIG. 11, thesemi-insulating substrate 81 is prepared which has a front surface and arear surface directed upwards and downwards in FIG. 11, respectively.

The buffer layer 83 of undoped In₀.52 Al₀.48 As is formed on theprincipal surface 82 to the thickness of 1 μm by the use of MolecularBeam Epitaxy (MBE). A first film is deposited on the buffer layer 83 tothe thickness of 200 angstroms by MBE. The first film is composed of ann-type In₀.52 Al₀.48 As of which a doping density is 1×10¹⁹ /cm³.Sequentially, a second film of undoped In₀.52 Al₀.48 As is formed on thefirst film by MBE to the thickness of 50 angstroms. A third film ofundoped In₀.8 Ga₀.2 As is formed on the second film by MBE to thethickness of 80 angstroms. A fourth film of undoped In₀.52 Al₀.48 As isformed on the third film by MBE to the thickness of 50 angstroms. Afifth film is formed on the fourth film by MBE to the thickness of 200angstroms. The fifth film is composed of an n-type In₀.52 Al₀.48 As ofwhich a doping density is 1×10¹⁹ /cm³.

With Electron Beam lithography, the first through the fifth films areetched by the use of a mask which has a prescribed pattern. The firstfilm is etched into the first supplying film 84a having a plate portion841 and a projection portion 842 on a center of the plate portion 841.The second film is etched into the first spacer film 85a on theprojection portion 842. The third through the fifth films are etchedinto the quantum well layer 86, the second spacer film 85b, and thesecond supplying film 84b, respectively. As described above, the quantumwell layer 86 has a quantum wire structure of which the crystalorientation is parallel to (100) orientation and of which the width is160 angstroms.

When applying the electric field to the semiconductor device 80, theone-dimensional electron gas is generated in the quantum well layer 86and will be designated by a reference numeral 87 in FIG. 11.

As readily understood from description in conjunction with FIG. 11, thequantum well layer 86 serves as the first semiconductor layer. Theelectron supplying layer 84 serves as the second semiconductor layer incooperation with the spacer layer 85.

Referring to FIG. 12, description will proceed to a potential energylevel in relation to a direction perpendicular to the quantum wirestructure of the semiconductor device 80 illustrated in FIG. 11. In FIG.12, E.sub.Γ^(W), E_(L) ^(W) and E_(X) ^(W) represent the first Γ-valley,the first L-valley, and the first X-valley potential energy levels,respectively, as described in conjunction with FIG. 2. SimilarlyE.sub.Γ^(B), E_(L) ^(B), and E_(X) ^(B) represent the second Γ-valley,the second L-valley, and the second X-valley potential energy levels,respectively.

Now, let m*Γ be 0.032m₀ in Equation (1). Inasmuch as the width Ly andthe thickness Lz are equal to 80 angstroms and 160 angstroms in thequantum wire structure, respectively, the ground level ε₁₁ becomes to0.23 eV in the first Γ-valley. The first excitation level ε₁₂ becomes to0.36 eV in the first Γ-valley.

As shown in FIG. 12, the first Γ-valley potential energy level is lowerin 0.66 eV than the first L-valley potential energy level. The secondΓ-valley potential energy level is lower in 0.22 eV than the secondL-valley potential energy level. The first Γ-valley potential energylevel is lower in 0.70 eV than the second Γ-valley potential energylevel at the heterojunction surface between the quantum well layer 86and the electron supplying layer 84.

Accordingly, the first Γ-valley has at least one one-dimensionalsubband. In the example being illustrated in FIG. 12, the first Γ-valleyhas a pair of one-dimensional subbands of which levels are depicted byreference numbers 91 and 92, respectively. In the one-dimensionalsubband having the level depicted by 91, electrons have a wave functiondepicted by a reference number 93. In the one-dimensional subband havingthe level depicted by 92, electrons have a wave function depicted by areference number 94.

Similarly, the first L-valley has at least one one-dimensional subbandinasmuch as the first L-valley potential energy level is lower than thesecond potential energy level. In the example being illustrated in FIG.12, the first L-valley has a pair of one-dimensional subbands of whichlevels are depicted by reference numbers 95 and 96, respectively. In theone-dimensional subband having the level depicted by 95, electrons havea wave function depicted by a reference number 97. In theone-dimensional subband having the level depicted by 96, electrons havea wave function depicted by a reference number 98.

As readily understood from the above description, the electron supplyinglayer 84 serves as a quantum barrier layer in cooperation with thespacer layer 85 in the fourth embodiment.

Now, it will be assumed that the quantum well layer 86 is composed ofIn_(x) Ga_(1-x) As. Equation (3) is satisfied when the variable x is notless than 0.7. In the quantum well layer 86 composed of In_(x) Ga_(1-x)As, a misfit dislocation occurs in the quantum well layer 86 when thethickness of the quantum well layer 86 is 100 angstroms at the variablex=0.8. The misfit dislocation occurs in the quantum well layer 86 whenthe thickness of the quantum well layer 86 is 50 angstroms at thevariable x=1. In Equation (1), it will be assumed that the width Ly andthe thickness Lz are equal to each other. When each of the width Ly andthe thickness Lz is equal to 50 angstroms, the ground state level isestimated as 1.1 eV in the first Γ-valley in case where m*Γ is equal to0.032 m₀. When each of the width Ly and the thickness Lz is equal to 100angstroms, the ground state level is estimated as 0.28 eV in the firstΓ-valley in case where m*Γ is equal to 0.032 m₀.

As described in conjunction with FIG. 12, the first L-valley potentialenergy level is higher in 0.66 eV than the first Γ-valley potentialenergy level when the quantum well layer 86 is composed of In₀.8 Ga₀.2As. Therefore, the variable x should not be greater than 0.8 in thequantum well layer 86 to make the semiconductor device 80 maintain aone-dimensional carrier and to make the semiconductor device 80 be adirect transition type.

While this invention has thus far been described in conjunction withpreferred embodiments thereof, it will readily be possible for thoseskilled in the art to put this invention into practice in various othermanners. For example, the first and the second semiconductor layers maybe composed of various other materials, respectively. Although ElectronBeam lithography is used in the first through the fourth embodiments onforming the quantum wire structure, another process, for example,Fractional Layer Superlattice Epitaxy may be used on forming the quantumwire structure.

What is claimed is:
 1. A semiconductor device having a quantum wirestructure formed by first and second semiconductor layers, said quantumwire structure being extended towards a predetermined direction, saidsecond semiconductor layer comprising:a first electron supplying layer;and a second electron supplying layer; said first semiconductor layerbeing interposed between said first and said second-electron supplyinglayers: said first semiconductor layer having a first conduction bandwhich has a first Γ-valley and a first L-valley, said first Γ-valleyhaving a first Γ-valley energy level, said first L-valley having a firstL-valley energy level which is not lower than said first Γ-valley energylevel: said second semiconductor layer having a second conduction bandwhich has a second energy level which is defined by an energy level ofthe bottom of said second conduction band and which is higher than saidfirst L-valley energy level: said predetermined direction being parallelto a prescribed crystal orientation.
 2. A semiconductor device asclaimed in claim 1, wherein:said first semiconductor layer is used as aquantum well layer; and said second semiconductor layer being used as aquantum barrier layer.
 3. A semiconductor device as claimed in claim 1,wherein said prescribed crystal orientation is (100) orientation.
 4. Asemiconductor device as claimed in claim 1, wherein:said firstsemiconductor layer is composed of InGaAs; and said second semiconductorlayer being composed of AlGaAs.
 5. A semiconductor device as claimed inclaim 4, wherein:said first semiconductor layer is an undoped layer; andsaid second semiconductor layer being further composed of an impuritywhich is doped in said second semiconductor layer.
 6. A semiconductordevice as claimed in claim 4, further comprising a semi-insulatingsubstrate and a buffer layer formed on said semi-insulating substrate,wherein:said second semiconductor layer is formed on said buffer layer.7. A semiconductor device as claimed in claim 6, wherein:saidsemi-insulating substrate is composed of GaAs which is undoped; and saidbuffer layer being composed of GaAs which is undoped.
 8. A semiconductordevice as claimed in claim 5, wherein said impurity is an n-typeimpurity.
 9. A semiconductor device as claimed in claim 5, wherein:saidfirst semiconductor layer has a first composition which is given byIn_(x) Ga_(1-x) As, where x is a first variable between 0.1 and 0.3(both inclusive); and said second semiconductor layer has a secondcomposition which is given by Al_(y) Ga_(1-y) As, where y is a secondvariable between 0.35 and 0.55 (both inclusive).
 10. A semiconductordevice as claimed in claim 1, wherein:said first semiconductor layer iscomposed of InAs; and said second semiconductor layer being composed ofAlGaSb.
 11. A semiconductor device as claimed in claim 10, wherein:saidfirst semiconductor layer is an undoped layer; and said secondsemiconductor layer being further composed of an impurity which is dopedin said second semiconductor layer.
 12. A semiconductor device asclaimed in claim 10, further comprising a semi-insulating substrate, asuperlattice layer formed on said semi-insulating substrate, and abuffer layer formed on said superlattice layer, wherein: said secondsemiconductor layer is formed on said buffer layer.
 13. A semiconductordevice as claimed in claim 12, wherein:said semi-insulating substrate iscomposed of GaAs which is undoped; said superlattice layer beingcomposed of GaSb/AlSb which is undoped; and said buffer layer beingcomposed of AlGaSb which is undoped.
 14. A semiconductor device asclaimed in claim 11, wherein said impurity is an n-type impurity.
 15. Asemiconductor device as claimed in claim 11, wherein said secondsemiconductor layer has a composition which is given by Al_(y) Ga_(1-y)Sb, where y is a variable between zero and one (both inclusive).
 16. Asemiconductor device as claimed in claim 15, wherein said variable y isequal to one.
 17. A semiconductor device as claimed in claim 1,wherein:said first semiconductor layer is composed of GaAs; and saidsecond semiconductor layer being composed of InAlGaP.
 18. Asemiconductor device as claimed in claim 17, wherein:said firstsemiconductor layer is an undoped layer; and said second semiconductorlayer being further composed of an impurity which is doped in saidsecond semiconductor layer.
 19. A semiconductor device as claimed inclaim 17, further comprising a semi-insulating substrate, wherein:saidsecond semiconductor layer is formed on said buffer layer.
 20. Asemiconductor device as claimed in claim 19, wherein:saidsemi-insulating substrate is composed of GaAs which is undoped; and saidbuffer layer being composed of GaAs which is undoped.
 21. Asemiconductor device as claimed in claim 18, wherein said impurity is ann-type impurity.
 22. A semiconductor device as claimed in claim 21,wherein said second semiconductor layer comprises:a main film composedof InAlGaP; and an additional film located in said main film, saidadditional film including said impurity.
 23. A semiconductor device asclaimed in claim 22, wherein said impurity is silicon.
 24. Asemiconductor device as claimed in claim 18, wherein said secondsemiconductor layer has a composition which is given by In₀.49 (Al_(z)Ga_(1-z))₀.51 P, where z is a variable between 0.4 and 1 (bothinclusive).
 25. A semiconductor device as claimed in claim 1,wherein:said first semiconductor layer is composed of InGaAl; and saidsecond semiconductor layer being composed of InAlAs.
 26. A semiconductordevice as claimed in claim 25, wherein:said first semiconductor layer isan undoped layer; and said second semiconductor layer being furthercomposed of an impurity which is doped in said second semiconductorlayer.
 27. A semiconductor device as claimed in claim 25, furthercomprising a semi-insulating substrate and a buffer layer formed on saidsemi-insulating substrate, wherein:said second semiconductor layer isformed on said buffer layer.
 28. A semiconductor device as claimed inclaim 27, wherein:said semi-insulating substrate is composed of InPwhich is undoped; and said buffer layer being composed of InAlAs whichis undoped.
 29. A semiconductor device as claimed in claim 26, whereinsaid impurity is an n-type impurity.
 30. A semiconductor device asclaimed in claim 26, wherein said second semiconductor layer comprises:amain film composed of InAlAs and said impurity; and a spacer filmcomposed of InAlAs which is undoped, said first semiconductor layerbeing interposed by said spacer film.
 31. A semiconductor device asclaimed in claim 26, wherein:said first semiconductor layer has a firstcomposition which is given by In_(x) Ga_(1-x) As, where x is a variablebetween 0.7 and 0.8 (both inclusive); and said second semiconductorlayer having a second composition which is given by In₀.52 Al₀.48 As.